When Viewfactor makes resistors from a surface to the ambient environment node, it calculates the view to the ambient environment by summing the viewfactors from the surface to all other surfaces in the enclosure and subtracting from one. If the opening in the enclosure is small, it is possible to obtain a small negative number by this procedure due to numerical and round-off error. This will result in a resistor with a negative value. This is not a problem for the network analyzer QTRAN and you should not become unduly alarmed about these negative valued resistors. They are retained because they are the best approximation of the correct value.